Four years since it disappeared, the reason Malaysia Airlines flight MH370 left its planned flightpath and headed towards the Indian Ocean is still one of aviation’s most perplexing mysteries.

The Malaysian government has prepared a report on potential causes of the disappearance, but it will not be released until the latest search – by the ship Seabed Constructor – finishes in June.

In the vacuum of information, theories – some more likely than others – have sprung up. These are the four main contenders:

Mass hypoxia event

The official theory, adopted by both the Malaysian government and the Australian Transport Safety Bureau, is that the passengers and crew of MH370 were incapacitated by an unknown “unresponsive crew/ hypoxia event”. Hypoxia is a deficiency of oxygen.

MH370 disappeared on 8 March 2014, en route from Kuala Lumpur to Beijing. Two hours after takeoff, military radar showed it had abruptly left its planned route, turned around and flown back towards Malaysia.

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The plane left radar screens and continued flying for another six hours, making satellite contact – known as a “handshake” – seven times. The final handshake, made as the plane ran out of fuel, most likely placed MH370 in the south Indian Ocean near Australia. An eighth attempt, made an hour later, received no response, meaning the plane had crashed.

It is what happened in this hour that is unclear.

The official opinion is that the captain, Zaharie Ahmad Shah, was unconscious and had been for hours. As MH370 ran out of fuel, it flew on autopilot and finally crashed into the sea.

Other hypoxia theories include a deliberate hijacking, a wrestle over the controls (both of which could cause hypoxia through flying steeply upwards), or another kind of mechanical accident.

The Seabed Constructor in the search for MH370. Photograph: Tony Ashby/AFP/Getty Images

Fire or accident

In the immediate aftermath of the disappearance, former pilot Christopher Goodfellow speculated that an electrical fire broke out on board. He said this explained the first turn towards Malaysia as Shah was searching for an emergency landing strip. He believes the fire then incapacitated Shah and the cabin crew, leaving the plane to fly south on autopilot.

Patrick Smith, another pilot, has cast doubt on the fire theory, saying it was unlikely MH370 could have continued for six hours on autopilot after a major fire. Officials believe Shah was unconcious, but have not offered any theories as to why or when this occurred.

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A key issue is the fact that MH370 turned twice: once towards Malaysia and then toward the Indian Ocean. If Shah was unconscious at the end, he or another pilot would have had to have made the second turn before losing control and set an autopilot course south themselves.

The rogue pilot

Byron Bailey, a former RAAF trainer and captain with Emirates, believes the plane was under the control of its captain as part of a deliberate descent into the Indian Ocean.

This would radically alter the current search operation – and potentially explain why the plane has not been found. Current and previous searches assumed the plane dived steeply and suddenly, with nobody at the helm, near the location of the seventh handshake.

Zaharie would have depressurised the cabin to put everyone else to sleep and then death

Byron Bailey

But if Shah was conscious, he could have manoeuvred the plane in a long, slow glide, travelling almost 200km further south. This also would have kept the plane more intact, with less debris.

Bailey believes Shah depressurised the plane to incapacitate his passengers and crew, before flying the plane solo for the next six hours until its fuel ran out.

“Having sent the co-pilot to the passenger cabin on an errand early on, Zaharie would have depressurised the cabin to put everyone else to sleep and then death … [while] he enjoyed his longer supply [of air],” Bailey wrote.

“With everyone dead, Zaharie would have repressurised the aircraft and flown the rest of the trip in comfort.”

In 2016, the ATSB chief commissioner at the time, Martin Dolan, acknowledged a rogue pilot theory could be considered if the search continued to fail to find the plane.

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But later that year, new data prompted an ATSB report to conclude a “high and increasing rate of descent” meant the plane was out of control in its final moments – making an uncontrolled descent the official explanation.

In the ATSB’s final report, published in 2017, it further refuted the possibility of a conscious pilot, saying the pattern of scattered debris meant “there was a significant amount of energy at the time the aircraft impacted the water, not consistent with a successful controlled ditching”. A section of the plane’s flap was also “in a retracted position”, which made a controlled descent “very unlikely”, it said.

A northern landing

Yet another theory says the plane is not near Australia at all, but rather to the north of Malaysia.

This theory stems from the way satellite data is calculated. After MH370 turned back towards Malaysia, its last known military radar point showed it travelling slightly north-west towards India.

From there investigators were forced to rely on satellite data alone to gauge its location. Unlike radar, satellite data cannot pinpoint a plane’s position, but places it in an estimated range that forms a circle, with the satellite at the centre.

Thus it is technically possible that MH370 travelled north inland, rather than south across the ocean. Some speculate the plane crashed, or was piloted to a landing and hidden somewhere in central Asia.

If they did not know who this plane was, I’d expect them to scramble jets to find out

Ron Ruggeri

However, the existence of a northern location was ruled out by Inmarsat, the British company that owned and operated the satellite that tracked MH370. The company has stated its analysis is consistent only with a southern journey into the Indian Ocean.

In 2015, a flaperon from MH370 was found on Reunion Island off the coast of east Africa, followed by more confirmed debris from the plane. Drift modelling suggests this could only occur if the plane crashed in the Indian Ocean.

Another issue is the fact that MH370 would have had to fly for six hours over populated and heavily militarised central Asian countries like Kazakhstan and Kyrgyzstan, and would have been spotted on military radar. Both governments say they did not detect the plane.

Ron Ruggeri, an experienced air traffic controller summed up the situation to the Guardian in 2014: “Things are also a little on edge in that part of the world. If they did not know who this plane was, I’d expect them to scramble jets to find out.”

Photo evidence

Other armchair investigators have claimed to have discovered photo evidence of debris that places MH370 in various other locations, but all have been discredited.

On Monday, Peter McMahon, an Australian investigator, told the Daily Star he had discovered the plane on Google Maps near Mauritius and submitted photo evidence to the ATSB.

But the ATSB pointed out his images were more than 10 years old and predated the plane’s disappearance.

“The images sent to ATSB by Mr McMahon were captured on 6 November 2009, more than four years before the flight disappeared,” a spokesperson for the ATSB said.